The principal object of the present invention is to provide an improved fusible element for an electric time-lag motor starter fuse having a voltage rating up to 600 volts and also capable of current-limiting action.
U.S. Pat. No. 3,935,553; 01/27/76 to Frederick J. Kozacka et al, CARTRIDGE FUSE FOR D-C CIRCUITS describes a time-lag fuse having a fusible element capable of meeting the above requirements; but which is difficult and expensive to manufacture.
It is, therefore, another object of this invention to provide an improved version of the fusible element and of the fuse described in the above patent.
The family of patents recited below is of interest in connection with the present invention. Each of them describes a fuse comprising a fusible element having a perforated center portion having an M-effect overlay, and non-perforated end or heat dam portions of smaller cross-section than the center portion and connecting the center portion and the terminal elements, or terminal caps, of the fuse.
U.S. Pat. No. 3,261,950; 07/19/66 to F. J. Kozacka for TIME-LAG FUSES HAVING HIGH THERMAL EFFICIENCY; U.S. Pat. No. 3,261,952; 07/19/66 to F. J. Kozacka for TIME-LAG FUSE WITH RIBBON FUSE LINK HAVING TWO SYSTEMS OF BENDS; U.S. Pat. No. 3,291,943; 12/13/66 to F. J. Kozacka for TIME LAG FUSE WITH RIBBON FUSE LINK FOLDED IN LONGITUDINAL AND IN TRANSVERSE DIRECTION; U.S. Pat. No. 3,319,028; 05/09/67 to F. J. Kozacka for SPRINGLESS TIME LAG FUSE FOR MOTOR CIRCUITS; U.S. Pat. No. 3,341,674; 09/12/67 to P. C. Jacobs, Jr. for ELECTRIC QUARTZ-SAND-FILLED FUSE ADAPTED TO INTERRUPT EFFECTIVELY PROTRACTED SMALL OVERLOAD CURRENTS; and U.S. Pat. No. 3,382,335; 05/07/68 to F. J. Kazacka for ELECTRIC FUSE HAVING PRISMATIC CASING.
The points of reduced cross-section of a fusible element for a current-limiting fuse should be of silver to minimize the fusing i.sup.2 .multidot.t thereof. There is no such requirement in regard to the heat dam portions of the fusible element. It is, therefore, another object to provide fusible elements whose points of arc initiation are of silver, but whose heat dams are of a less expensive metal other than silver.
Great difficulties arise when blanking or stamping the center portion and the heat dam portions out of a piece of metal and folding the center portion and the heat dam portions in opposite directions, as required by the above prior art patents. In particular, when folding the heat dam sections in transverse direction, as required, both ends of the heat dam sections tend to move in a direction longitudinally thereof, but are only allowed at their axially outer ends to do so while their axially inner ends are fixed by the perforated center section or center portion of the fusible element.
It is, therefore, a further object of the invention to greatly facilitate the manufacture of fusible elements including a relatively wide perforated center section folded in a direction longitudinally thereof and relatively narrow heat dam sections folded in a transverse direction.
Other objects of the invention will become more apparent as this specification proceeds.